The present invention is intended to measure various geometric, positional and kinematic parameters with respect to and during the rotation of a rotatable device, such as a disk or drum, about a rotation axis where it is desired to utilize such parameters to control various functions of the device. The present invention is particularly applicable to computerized tomography (CT) scan systems and will therefore be described particularly in that context, but should not be considered to be limited to such systems. CT scan systems typically include a gantry comprising a disk or drum rotatable within a frame. In third generation CT scanners an X-ray source and X-ray detector array are mounted on the disk for rotational motion therewith about a table on which a patient can repose. The X-ray source and X-ray detector array are positioned about a point on the disk that defines the locus, hereinafter referred to as the "geometric center", about which the source and detector array prescribe correct rotational movement when the disk is rotated about the point during a scan so that the tomographic image can be accurately reconstructed. This geometric center ideally coincides with the nominal center of mass of the disk as well as the rotational center of the disk. In fourth generation CT scanners the X-ray source is mounted on a rotatable disk relative to the geometric center, while the detectors are disposed on the stationary frame equiangularly about the rotation axis of the disk. In both types of systems, the X-ray source may provide periodic pulses or continuous wave radiation. Each detector typically is either a solid state or a gas tube device.
In third generation type machines the detector array is disposed diametrically opposite the source across the disk, and is aligned with the focal spot of the source so that the detector array and focal spot are positioned within a common mean, scanning or rotation plane (normal to the axis of rotation of the disk). In the case of a detector array, each detector of the array is positioned in the scanning plane, typically at a predetermined angular spacing relative to the source so that each detector subtends an equal angle relative to the focal spot, thus providing a plurality of different X-ray paths in the scanning plane between the source and the respective detectors. In third generation machines, the X-ray paths can collectively resemble a fan with the apex of the fan at the focal spot of the X-ray source. In fourth generation machines the detectors are mounted on the gantry frame and the X-ray paths with respect to each detector resembles a fan with the apex at each detector input. Accordingly, both types of machines are sometimes referred to as "fan beam" tomography systems.
These systems provide a plurality of information or data signals corresponding to variations in the radiation flux measured by the detectors at each of a plurality of projection views, i.e., at precise angular positions of the disk during rotation of the disk about an object occupying the space between the detectors and the X-ray source. Upon known (Radon) mathematical processing of the signals commonly referred to as "back projection", a visual image can be formed representing a two-dimensional slice along the plane of rotation, i.e., the scanning plane, through the portion of the scanned object positioned in the plane between the source and the detectors. For helical scanned images, the object being scanned and the rotating disk are moved relative to one another in the direction of the rotation axis, while data is acquired. Other types of scanned images have also been proposed. See for example, U.S. patent application No. 08/831,558, entitled Nutating Slice CT Image Reconstruction Apparatus and Method, filed Apr. 9, 1997, in the names of Gregory L. Larson, Christopher C. Ruth, and Carl R. Crawford, and assigned to the present assignee, the application being incorporated herein by reference (Attorney's Docket No. ANA-118). The accurate formation of such images critically depends upon various factors including: (1) the movement of the disk being rotational precisely about the geometric center of the disk, (2) the geometric center remaining fixed in the scanning plane during a scan so that it does not move laterally within the plane relative to the scanned object as the disk rotates about its axis, (3) the X-ray exposure provided during each projection view being the same for each view, and (4) data being taken at precise angular positions of the disk so that data is correlated with the correct positional information of the X-ray source and/or the detectors relative to the scanned object when the image is back projected. The present invention is directed to improvements in the fourth factor, i.e., data being taken at precise angular positions, although those skilled in the art will recognize that parameters measured by the invention may also be used to improve the other three aforementioned factors.
A system for and method of measuring any one of several geometric, positional and kinematic parameters relating to a rotating device was described and claimed in U.S. Pat. No. 5,432,339, entitled APPARATUS FOR AND METHOD OF MEASURING GEOMETRIC, POSITIONAL AND KINEMATIC PARAMETERS OF A ROTATING DEVICE HAVING A PLURALITY OF INTERVAL MARKERS, which is hereby incorporated by reference, and is referred to as the "'339 Patent" herein. As described in the '339 Patent a measurement and control system can use the measurements to control the operation of various components on the rotating device.
A disadvantage inherent in the system described in the '339 Patent is that parametric data are collected by sensors which are stationary with respect to the rotating device. When components located on the rotating device must have access to the parametric data, the measurement and control system communicates the parametric data to the rotating device typically through communication means such as slip rings or broadcast (rf) transmission means. Such communications means increases the overall system cost, and can add noise and time delays to the parameter measurements.
It is therefore an object of this invention to provide a system which collects parametric data related to a rotating device while the system is resident on the rotating device, thereby eliminating the need to communicate such data to the rotating device.
Another object of this invention is to provide a system which generates a trigger signal as a function of parametric data related to a rotating device.
Yet another object of this invention is to provide a system which provides a modulation signal, possibly to a power supply so as to generate a power supply output voltage pattern as a function of parametric data related to a rotating device.
Other objects of the present invention will in part be evident and will in part appear hereinafter. The invention accordingly comprises the process involving the several steps and the relation and order of one or more of such steps with respect to the others and the apparatus possessing the construction, combination of elements, and arrangement of parts exemplified in the following detailed disclosure, and the scope of the application of which will be indicated in the claims.